In RF discharge energized slab CO2 lasers an arrangement (pre-ionizer) for pre-ionizing the lasing gas to facilitate igniting the RF discharge is often provided. A slab laser usually has a sealed enclosure filled with the lasing gas. The lasing gas pressure is usually between about 50 and 150 Torr. A pair of elongated slab electrodes is arranged in the enclosure with the slab electrodes parallel to each other and spaced apart defining a discharge gap between the electrodes. The discharge gap is filled with the lasing gas of the enclosure. Applying RF power to the slab electrodes ignites and sustains a discharge in the discharge gap, thereby energizing the lasing gas and causing the laser to lase. In the absence of a pre-ionizer, the time required to ignite the discharge between the slab electrodes can vary randomly, or in order to ignite the discharge, it may be necessary to increase the RF power to a level two or more times greater than the power necessary to sustain the discharge once it is ignited.
In one preferred prior-art pre-ionizer, a pair of closely spaced metal pre-ionizer electrodes or pins, is located in the enclosure in which the main (slab) electrodes are located. The electrodes are typically spaced by between about 0.5 and 1.0 millimeter (mm). The pre-ionizer electrodes are significantly smaller than the slab electrodes, and are located near a wall of the enclosure remote from the discharge gap. RF power is applied constantly to the pre-ionizer electrodes and creates a local discharge around the pre-ionizer electrodes, this discharge being of small volume compared to the volume of the enclosure. The close spacing is required to ensure that the pre-ionizer discharge itself ignites reliably. The pre-ionizer discharge provides ions that diffuse throughout the volume of laser gas in the enclosure. These ions are generated directly by the local discharge, and indirectly generated by interaction of the lasing gas with fluorescence radiation (particularly ultraviolet radiation) emitted by the discharge. The presence of these ions in the lasing gas volume facilitates igniting the lasing gas discharge between the slab electrodes when the main RF power is applied to the electrodes, and can allow essentially instantaneous ignition of the discharge. This reduces the excess power that needs to be applied to ignite the main discharge.
A problem with prior art pre-ionizing arrangements including such closely spaced metal electrodes is that these metal electrodes generate particulate matter during operation over thousands of hours. This particulate matter is generated, inter alia, by sputtering and corrosion of the pre-ionizer electrodes and can be harmful inside the lasing gas enclosure. There is a need for a pre-ionizing arrangement that does not generate such particulate matter.